1. Field of the Invention
The present invention relates to circuits for converting received radio frequency signals (“RF signal”, see definitions section) signal into modified signals (see Definitions section) and more particularly to radar receiver circuitry for converting received radar RF signals into modified signals useful in generating radar information.
2. Description of the Related Art
It is conventional to convert radio waves into electrical RF signals. Radios, radar, cell phones and other wireless communication systems all do this. It is conventional to convert RF signals into modified RF signals by the use of modifying devices. One example of this is when a received RF signal is processed by a low noise amplifier modifying device. Another example of this is when an RF signal is processed by a modifying device into an intermediate frequency (“IF”) electrical signal. A couple of conventional circuits for processing RF signals will now be discussed.
U.S. Pat. No. 6,191,725 (“Lavoie”) at FIG. 1 (“FIG. 1 Lavoie”) discloses an automatic gain control circuit for electromagnetic wave receiving systems, such as radar warning receivers. In FIG. 1 Lavoie, a radar pulse is received, presumably as an RF signal (Lavoie does not disclose this part of the FIG. 1 Lavoie system in detail). The RF signal is converted to an IF signal, presumably by modifying it with a low noise amplifier (“LNA”) and/or other appropriate modifying device(s) (again, Lavoie does not disclose this part of the FIG. 1 Lavoie system in detail). The converted IF signal of Lavoie then passes through an analog delay line to a fast programmable attenuator, and then to an amplifier. The fast programmable attenuator of FIG. 1 Lavoie is controlled by control circuitry based on a comparison of amplitude of the received IF signal to an appropriate reference level. The analog delay line provides the programmable attenuator with time to settle. It is noted that the programmable attenuator of FIG. 1 Lavoie does not provide protection to circuit components upstream of the programmable attenuator, such as the modifying device(s) that would be required for FIG. 1 Lavoie to convert a received RF pulse into a corresponding IF pulse. Also, the programmable attenuator of FIG. 1 Lavoie attenuates solely on the basis of the amplitude of the IF signal, and does not account for the frequency or duration of a pulse present in the IF signal.
U.S. Pat. No. 7,088,794 (“Nichols”) discloses an automatic gain control RF signal processor for receiver systems, such as radar intercept receivers. As shown at FIGS. 1 and 2 of Nichols, in the Nichols circuitry, an electrical signal passes through the following devices in the following order: (i) any modifying device(s) necessary to convert the received RF signal into an analog IF input signal (see left hand side of FIG. 1 of Nichols); (ii) an attenuator; (iii) a fixed amplifier; (iv) a bandpass filter; (v) an analog to digital converter; and (vi) a digital delay. The digital delay is used so that the front edge of the signal is not missed due to latency in the decision-making process within the threshold and control logic. It is noted that the delayed portion of the signal that passes through the digital delay is not attenuated after it has been delayed because the attenuator is upstream of the delay in the Nichols circuitry. It is further noted that the modifying device(s) needed to converted the RF signal presumably received in Nichols into the analog input signal shown at the left side of FIG. 1 in Nichols are not protected by the attenuator of Nichols because these devices would be upstream of the attenuator and therefore not subject to the signal attenuation it provides.
Description Of the Related Art Section Disclaimer: To the extent that specific publications are discussed above in this Description of the Related Art Section, these discussions should not be taken as an admission that the discussed publications (for example, published patents) are prior art for patent law purposes. For example, some or all of the discussed publications may not be sufficiently early in time, may not reflect subject matter developed early enough in time and/or may not be sufficiently enabling so as to amount to prior art for patent law purposes. To the extent that specific publications are discussed above in this Description of the Related Art Section, they are all hereby incorporated by reference into this document in their respective entirety(ies).